Semiconductors

A semiconductor's electrical conductivity is intermediate between that of a metal and that of an insulator; the metalloids silicon and germanium are the classic examples. As in an insulator, the valence band of a semiconductor is completely filled and its conduction band is empty -- but the energy gap between the two bands is much smaller (Fig. 1.25). At any temperature above absolute zero, some electrons of the valence band gain enough thermal energy to jump this small gap into the conduction band: the conduction band becomes partially filled, the valence band becomes partially empty, and the material conducts a small amount of electricity when a potential is applied. A pure semiconductor showing this low but finite conductivity is called an intrinsic semiconductor.

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